1. Field of the Invention
The present invention relates to an apparatus and method for measuring a thin film or layer thickness and, more particularly, to an electro-optical system which measures the thickness of an outer silicon layer of a silicon/silicon dioxide/silicon (Si/SiO.sub.2 /Si) structured semiconductor wafer.
2. Description of the Prior Art
In one particular application wherein the present invention is especially practical, a silicon-on-insulator (SOI) semiconductor wafer typically includes a Si/SiO.sub.2 /Si sandwich structure fabricated by growing a silicon dioxide film on one surface of each of two silicon wafers and bonding the two silicon dioxide film surfaces together at high temperature. It will be understood that other materials such as, for example, silicon nitride, may be used for the insulator material and that other materials may be used for the wafer material. In such an application, one of the two outer silicon surfaces of the sandwich structure is mechanically ground and polished to an average thickness of several microns. This mechanical process unfortunately results in large spatial variations in the thickness of this outer silicon layer over the surface of the wafer. To reduce these spatial variations, a thickness error map that indicates thickness non-uniformities of this outer silicon layer over the entire wafer surface, is required, for example, to initialize a further micropolishing process.
A sequence of measuring the spatial variations in the thickness of the outer silicon layer followed by thinning and smoothing this surface by micropolishing may need to be performed several times before the entire outer silicon layer achieves the desired thickness. In order to reduce costs and increase production, a measurement of at least 400 points on a wafer surface in 60 seconds is desirable.
Current commercial instruments, however, typically provide film thickness measurements at only a single point on a surface. These instruments use a focused lens or a fiber bundle to locally illuminate the film surface with a beam of monochromatic light, and a grating or prism spectrograph to measure the surface spectral reflectance at each point. In all cases, this surface spectral reflectance data must be numerically corrected due to variations in the angle of incidence caused by the illuminating beam f-number.
These commercial instruments may be extended to cover an entire wafer surface by moving either the measuring instrument or the wafer in a controlled manner. However, the time required for these instruments to determine the thin film layer thickness at a single point is on the order of several minutes and characterizing an entire film surface of at least 400 measurement points far exceeds the time desired for efficient wafer production.